By IANS, [RxPG] History and geography shape our genome, according to a new study.

The movements of humans within and among continents, expansions and contractions of populations and vagaries of genetic chance, have influenced the distribution of genetic variations.

In recent years, geneticists have identified a handful of genes that have helped human populations adapt to new environments within just a few thousand years - a strikingly short time scale in evolutionary terms.

However, a team from the Universities of Chicago, California and Stanford, which jointly conducted the study, found that for most genes, it can take at least 50,000-100,000 years for natural selection to spread favourable traits through a human population.

They found that gene variants tend to be distributed throughout the world in patterns that reflect ancient population movements and other aspects of population history.

'In addition to selection, demographic history -- how populations have moved around -- has exerted a strong effect on the distribution of variants,' he added.

Selection may still be occurring in many regions of the genome, said Pritchard. But if so, it is exerting a moderate effect on many genes that together influence a biological characteristic, according to a Howard Hughes release.

'We don't know enough yet about the genetics of most human traits to be able to pick out all of the relevant variation,' said Pritchard.

'As functional studies go forward, people will start figuring out the phenotypes - associated with selective signals,' said lead study author Graham Coop.

'That will be very important, because then we can figure out what selection pressures underlie these episodes of natural selection.'

The study was published in the Friday edition of the open-access journal PLoS Genetics.